Transmission system



May 12, 1942. R. H. HERRICK 2,282,404

TRANSMISSION SYSTEM Filed Aug. 14, 1939 i 5 Shee'bS-She'et l May 12, 1942.

vR. H. HERRICK TRANSMISSION SYSTEM Filed` Aug. 14, 1939 3 Sheets-Sheet 2 INVENTOR.

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Patented May 12,1942

TRANSMISSION SYSTEM Roswell H. Herrick, Oak Park, Ill., assignor to Associated Electric Laboratories, Inc., Chicago, Ill., a corporation of Delaware Application August 14, 1939, Serial No. 290,088

Claims.

The present invention relates generally to improvements in signal current transmission systems of the type in which signal controlled switching circuits are provided for partially or completely blocking, under certain conditions, certain of the'signal current channels included therein and, more particularly, to improvements in telephone substation circuits having incorporated therein coupled signal current channels for the transmission of incoming and outgoing signal currents.

In the usual telephone substation circuit, a hybrid system or antiside tone impedance network is provided for preventing signal currents developed during operation of the transmitter from being transmitted to the receiver and for preventing signal currentslincoming over the line extending to the substation from being transmitted to the transmitter. In this type of arrangement one of the factors which determines the efficiency of the side tone suppression is the impedance of the talking circuit established by way of nal currents transmitted in the opposite direction. In other words, conversation break-ins are positively prevented.

It is an object of the present invention, therefore, to provide improved telephone substation apparatus of the character described wherein the blocking of the signal current transmitting and wherein the incoming and outgoing signal curtwo connected subscribers lines. This impedance is not the same for any two different connections and, accordingly, the hybrid system of each substation is usually balanced to provide maximum side tone suppression eciency for average line conditions. In installations wherein amplification of the incoming and outgoing signal currents is required, theconditions of unbalance introduced into the substation circuit by the impedances of the lines over which an established connection extends may become intolerable. This is particularly true in substation installations provided in an exchange area where the subscribers lines are of widely diierent lengths. In order to obviate orminimize the singing which may result due to unbalance of the substation circuit occasioned by unfavorable line conditions, signal controlled switching means may be provided in the substation circuit for selectively blocking the signal transmission channels when not in use. In the usual arrangement of this character the transmission channels are completely blocked when not in use. More particularly, during intervals when the transmitting means of the substation is being used to trans- 'mit outgoing signal currents, the incoming signal current channel is rendered completely inactive. Conversely, during those periods when signal currents are incoming to the substation, the transmitting channel is rendered completely inactive. An arrangement of this character prevents any interruption of a speech train being transmitted in one direction in response to sigrent channels are normally active and the transmission efliciencies of the two channels are varied in' opposite senses by varying the plate or anode impedances of electron discharge control tubes respectively coupled thereto in response to signal currents traversing either of the two channels.

It is another object of the invention to provide a signal transmission system of the character described wherein the signal current transmission efciency or gain of each channel is varied inversely in accordance with the amplitude of signal currents impressed upon the input side thereof. Y

In each of the three illustrated embodiments of the invention there is provided a telephone substation circuit which is adapted to be connected to an associated line and comprises the usual transmitting and receiving means. The

. circuit also includes a transmission system which channel and a second leg shunting a portion of the channel. One leg of each of the two voltage` dividers is bridged by an adjustable impedance in the form of the space current path of an associated electron discharge tube, whereby the signal current transmission eiciencies of the two channels are variable with variations in the output electrode impedances of the respective associated tubes. The control electrodes of the two tubes are normally biased to potentials such that i the associated channels are both active, a control circuit being provided for varying the control electrode potentials in response to signal currents traversing one of the channels so that the gain or signal current transmission efficiency of the one channel is substantially increased and the signal current transmission eiiiciency of the other channel is substantially decreased. In one embodiment of the invention, the control circuit further includes apparatus comprising one of the two control tubes for varying the transmission efficiency of the channel in use inversely in accordance with the amplitude of the signal input thereto, thus providing automatic gain control action. More-particularly, automatic gain control is achieved by varying .the bias on the control tube associated with the channel in use to vary the output electrode impedance thereof so that the transmission efliciency of the channel is changed inversely in accordance with the signal input thereto.

Further features of the invention pertain to the particular arrangement of circuit elements whereby the above-mentioned and additional operating features are attained.

The novel features believed to be characteristic of the invention are set forth with particularity in theappended claims, The invention,both as to its organization and method of operation, to-

gether with further objects and advantages y thereof will best be understood byreference to the specification taken in connection with the accompanying drawings in which Figure 1 illustrates a substation circuit having incorporated therein certain of the features of the invention as briefly outlined above; Fig. 2 illustrates a modified form of the improved substation circuit; and Fig. 3 illustrates a modication of the circuit arrangement shown in Fig. 2.

Referring now more particularly to Fig. 1 of the drawings, the substation circuit there illustrated is adapted to be connected to a telephone line terminating at the terminals |00, which line may, for example, form a part of a conventional lautomatic or manual telephone system, in which case, the distant end thereof will terminate in a line circuit the character of which is determined by the character of the exchange. Brieiiy described, the substation circuit comprises a transmitter or microphone and receiving means in the form of a loud speaker |02 which are adapted respectively to be coupled to an antiside tone impedance network including a hybrid coil |03 and a balancing circuit |04 by means of signal transmission channels |05 and |06. Preferably, the loud speaker |02 is of the well-known moving coil type having embodied therein a voice or signal current coil |01 which is adapted to be energized by incoming signal currents transmitted thereto through the hybrid coil |03 and over the transmission channel |06. The outgoing signal current channel |05 comprises a vacuum tube amplifier |08 arranged to deliver its output to two additional stages of amplification A|09 and ||0, the last of which is arranged to deliver its output to the hybrid system |03 through a coupling transformer This channel further comprises a voltage divider in the form of a potentiometer resistor ||2 having a nonadjustable leg H2 serially included in -the channel, and a second leg H2. any portion of which may be bridged across the channel by manual adjustment of the tap I3. More specifically, the stage of amplification |09 comprises an electron discharge tube ||4 of the well-known pentode type which includes la cathode ||5and a control grid ||6 coupled to the outputcircuit of the amplifier |00 by means of a resistance capacitance coupling network including a condenser ||1 anda resistor ||8. The amplifying stage ||0 similarly comprises an electron, discharge tube ||9 having a cathode and a control grid |2| coupled to the output electrodes of the preceding tube ||4 through a resistance capacitance coupling network which includes the voltage dividing resistor ||2, a condenser |2| and a load resistor |22. The output electrodes of the tube |9 are coupled to the outgoing signal current terminals of the hybrid system |03 through the coupling transformer For the purpose of biasing the control grid ||6 of the tube ||4 negatively with respect to the associated cathode H5, there is included in the output circuit of this tube a cathode biasing ressitor |22' which is shun-ted by a signal current by-pass condenser |23. Similarly, a cathode biasing circuit comprising a resistor |24 shunted by a by-pass condenser |25 is provided in the output circuit of the tube ||9. Operating voltages are impressed upon the anode and screen electrodes of the two tubes ||4 and ||9 from a source of direct current, not shown, but having its positive terminal connected to the terminals marked +B. This source is by-passed for signal currents in the amplier stages |09 and ||0 by means of by-pass condensers |26 and |21, respectively.

The signal transmission channel |06 is coupled to the incoming signal current terminals of the vider in the form of a potentiometer resistor |3| having a leg |3|a serially included in the channel and a second leg |3|b, any portion vof which may be connected in shunt with the channel by manually adjusting the tap |32. More particularly, the amplifier stage |29 comprises an electron disc harge tube |33 of the well-known pentode type which includes a control grid |34 and a cathode |35 coupled to the secondary winding of the transformer |28, and output electrodes including the cathode |35 and an anode |36 coupled to the input circuit of the power amplifier |30 through a resistance capacitance network which includes the potentiometer resistor |3|, a condenser |31 and a load resistor |38. The positive terminal of the anode currentv supply source is connected to the anode |36 and the screen electrode ofthe tube |33 through the-terminal marked +B, this source being by-passed for signal frequency currents through the provision of a shunt connected condenser |39. For the purpose of biasing the control grid |34 of the tube |33 to the proper negative potential with respect to the cathode |35, there is included in the output circuit of the tube a cathode biasing resistor |40 which is shunted by a by-pass condenser |4 In order to control the signal transmission efiiciencies of the two channels |05 and |06, two control circuits |42 and |43 are provided. Since these control circuits are in all respects identical, only the circuit |43 has been illustrated in detail. Briefly described, this circuit comprises an amplifier |44 having its input terminals coupled to the channel |05 at a point just preceding the amplifier stage |09 and its output terminals coupled through a transformer |45 to a rectifying circuit which includes a dry disk rectifier |46, preferably of the copper oxide type, and a load rein shunt with the shunt leg pass condenser I is provided in the output cir-v cuit of the tube |49 and is connectedso that the voltage developed thereacross is arranged to control the potentials respectively impressed upon r the control electrodes of two electron discharge tubes |5| and |52. To this end, the voltage developed across the resistor |50 is impressed upon the control electrode of the tube |5| through a resistor |53 and upon the control electrode of the tube |52 through a resistor |54. The indicated control electrodes are normally biased to the desired negative potentials through the provision of small C batteries |55 and |56. The two tubes |5| and |52 are connected to control the gains or signal current transmission efficiencies of the two channels |06 and |05, respectively. More particularly, the output electrodes of the tube |5| are bridged across the shunt leg |3|b of the voltage divider |3| through a direct current blocking condenser |51. The output electrodes of the control tube |52, on the other hand, are bridged across the series leg H23 of the voltage divider ||2 through a pair of blocking condensers |58 and |59. In order effectively to isolate the output electrode impedances of the two tubes |5| and |52 anode current is delivered to these two tubes through high impedance iron core inductors |60vand |6|. The output electrode impedance of the tube |52 is further isolated through the provisionA of a third high impedance iron core inductor |32 which is included in the cathode leg of the output circuit of this tube. The last-.mentioned impedance element is chosen to present an extremely high impedance to signal currents, whereby the condenser |59 is prevented from effectively short-circuiting the shunt leg ||2t of the voltage divider ||2 insofar as signal currents traversing the channel |05 are concerned. This element is also constructed to have a very low direct current resistance whereby no substantial bias is impressed upon the cathode of the tube |52 due to the inclusion of the element in the cathode leg of the output circuit of this tube.

As indicated above, the two tubes |5| and |52 are arranged to control the transmission eiliciencies of the signal current channels |06 and |05, respectively. Thus, the output electrode impedance, that is, the impedance between the anode and cathode of the tube |5| is connected i |3|b of the voltage divider |3I. Accordingly, if the output electrode impedance of the tub'e |5| is high, the equivalent value of this impedance as combined with that of the shunt leg |3|b is relativelyhigh so that a relatively small portion of the available signal voltage impressed across the outer terminals of the voltage divider |3| by the amplier stage |29 appears across the series leg |3 la thereof. Hence, a relatively large portion of the available vsignal voltage appears across the shunt leg |3|b and is impressed across the input terminals of the power amplifier |30. Thus, the transmission eiliciency of the channel |06 or the gain therethrough is relatively h=gh `when the output electrode impedance of the' tube 5| as determined by the potential impressed upon the control electrode of this tube is high. When, however, this potential is adjusted or changed to lower the output electrode impedance of the tube |5|, the

the microphone |0I, from the sound waves impinging upon the diaequivalent impedance formed by the combination of this impedance andthe resistance of the shunt leg |3|b is lowered to produce a corresponding increase in the signal voltage drop across the series leg |3ls. As a consequence, a lesser percentage of the available signal voltage is im` pressed upon the input electrodes of the power amplier |30. Thus, the gain or transmission l efiiciency of the channel |05 is reduced when the output electrode impedance of the tube |5| is def creased. StatedA generally, the signal current transmission efficiency of the channel |06 is variable directly in accordance with variations in the output electrode impedance of the tubeA |5|.

Since the output electrodes of the tube |52 are bridged across the series leg H23 of the voltage divider H2, thee'ect of-changing this impedance is the exact reverse of that just described with reference to the tube |5I. More particularly, if

the output electrode impedance of the tube |52 isv increased from a normal value to a higher value the equivalent impedance of this output electrode impedance as combined with the resistance of the leg H2B is increased, whereby the signal current flow. through the shunt leg H2 of the voltage divider llll is decreased to produce a corresponding decrease in the signal voltage impressed across .the input electrodes of the amplier tube H9. On the other hand,v if the output electrode |52 is decreased from a impedance of the tube high value to a lower value; the signal current ow through the shunt leg H2 is increased to produce a corresponding increase in `the percentage of the available signal voltage which is impressed across the input electrodes vof the tube H9. Thus, when the of the tube |52 is increased the gain or transmission eiiiciency of the channel |05 is decreased,

whereas when the value of this impedance is decreased the gain or transmission eiciency of the channel |05 is increased. In other words, the signal current transmission eiilciency of the channel |05 is variable inversely in accordance with variations in the output electrode impedance of the tube |52.

Norma11y,tne two tubes asl and |52 are biased to a point slightly above anode current cut-ofi so that the output electrode impedances thereof are relatively high. With the two tubes biased in this manner the transmission eiciencies yol the two channels |05 and |06 may be changed at will by manually adjusting the taps I3 and |32 so that they are approximately equal or bear any.

desired ratio to each other. Preferably, however, the adjustment is such that both channels are normally capable of transmitting signal currents with reasonably good eillciency b'ut are insufficient to permit local oscillation of the circuit even when the hybrid system is greatly unbalanced. With the two channelsin this condition if a user of the substation apparatus speaks into the signal currents resulting phragm of the microphone are amplified by the tandem connected amplifier units |08, |09 and l0 and are delivered through the coupling transformer and the hybrid system |03 to the line terminating at the terminals |00. Due to the side tone suppressing action of the hybrid system |03 only a portion of these currents are transmitted through this system to the channel |06. If, however, the line conditions of the lines interconnecting the illustrated substation circuit with another distant .substation are such that the hybrid system is substantially unbalanced the mag output electrode impedance l network comprising a resistor |65 and a condenser |66. This circuit is provided with two this diiliculty. More particularly, the signal voltage impressed across the input electrodes of the amplifier tube 4 is also impressed across the input terminals ofthe amplier |44. The amplified signal currents are transmitted through the coupling transformer |45 to the rectifying circuit including the dry disk rectifier |46. The

arrangement is such that immediately a signal' voltage is impressed upon this circuit the condenser |48 is quickly charged to build up a biasing voltage across the resistor |41 which bucks down the negative bias voltage normally impressed between the control electrodes of the direct current amplier |49 bythe battery |46,

control tubes corresponding to the tubes |5I and |62 of the control circuit |43. AThe output electrodes of one of the two tubes are bridged across the series leg |3| ofthe voltage divider |3| through a pair of low impedance direct current blocking condensers |61 and |68. The output electrodes of the other control .tube embodied in the circuit |42 are bridged across the shunt leg ||2b o! the voltage divider ||2 through a blockthus permitting anode current to traverse the.,

output circuit of this amplifier, whereby a direct Voltage is developed across the load resistor |50. The rectifier |46 is so poled that it only passes current of a polarity which will produce a positive biasing voltage between the input electrodes of the amplifier tube |49. The pulsations of this voltage are'smoothed out by the action of the condenser |48.

The space current traversingthe tube |49 and the load resistor |50 as a result of the signal voltage impressed upon the input electrodes of the amplifier |44 results in a voltage drop across the resistor |50 which is impressed as a positive biasing potential upon the control electrodes of the tubes |5| and |52 through the resistors |53 and |54, respectively. Thus, the negative bias normally impressed upon the indicated control electrodes by the batteries |55 and |56 is decreased to produce a substantial decrease inthe output electrode impedances of these two tubes. As explained above, the decrease in the output electrode impedance of the tube |5| which shunts the shunt leg |3|b of the voltage divider |3| results in a substantial decrease in the gain or signal current transmission eiiiciency of the channel |06. Conversely, the decrease in the output electrode impedance of the tube |52 which shunts the seriesleg ||2a of the voltage divider ||2, results in a substantial increase in the transmission efficiency of the channel |05 over which the signal currents are being transmitted from the microphone |0| to the line connected to the terminals |00. By proper selection and adjustment,

of theV circuit elements involved, the transmission eiiiciency of the channel |06 may be decreased during use of the channel |05, to a point where no substantial reproduction of signal currents passed through the hybrid system |03 to the input side of the channel |06 occurs. The

ing condenser |69. With the control circuit |42 arranged in this manner it will be apparent from the preceding description with reference to the operation o! the circuit |43 that when signal currents are delivered from the line extending to the` terminals |00 through the hybrid system |03 to the channel |06 to cause the output electrode impedances of the tubes embodied in the control circuit |42 to be decreased, the transmission eiliciency of the channel |06 is decreased and the transmission efiiciency of. the channel |06 'is increased. Due to.- the decreased transmission eiliciency of the channel v|05, only a negligible por? tion of the sound energy is retransmitted through the acoustical coupling between the loud speaker |02 and the microphone |0|, the channel |05 and the hybrid system |03 to the line terminating at the terminals |00.

By virtue oi.' the particular arrangement of the transmission channels |05 and |06, the control circuits |42 and |43 and the hybrid system |03, it is possible for conversation break-ins to occur. Thus, if va user` of the substation a'pparatus illustrated is speaking into the microphone |0| so that signal currents are being transmitted through the channel |05 and the hybrid system |03 to the line terminating at the terminals |00, the transmission channel |05 is operating with a high transmission efciency, while the transmission eihciency of the channel |06 is quite low. Under these conditions if the party at the distant substation breaks in upon the conversation by causing signal currents to flow over the line terminating at the terminals |00 through the hybrid system |03 to the input side of the channel |06, the resulting operation of the control circuit |42 causes the transmission eniciency of the channel |05 to be lowered and the transmission efciency of the channel |06 to be raised. More particularly, the/signal currents incoming to the illustrated substation circuit are,

by virtue of the arrangement of the hybrid syssignal current transmission eflciencies of the two channels |05 and |06 are restored to normal in an obvious manner when the flow of signal currents over the channel |05 is terminated.

As explained previously, the control circuit |42 is identical in arrangement to the control circuit |43 and is provided for the purpose of controlling the transmission efficiencies of the two channels |05 and. |06 during those intervals when signal currents are transmitted to the substation circuit over the line extending to the terminals |00. More particularly, the input terminals of the control circuit |42 are coupled to the channel |06 at a point immediately following the stage of ampliilcation' |29 through a resistance capacitance tem |03 transmitted with much greater efficiency to the input side of the channel |06 than are the signal currents transmitted through this system from the channel |05 to the channel |06 due to the unbalanced condition thereof. Accordingly, the magnitude .of the incoming signal currents, when amplified by the amplifier unit |29 and impressed upon the input terminals of the control circuit |42, is suiilcient to cause the output electrode impedances of the two control tubes embodied in this control circuit to be substantially reduced. In response to the reduction of these impedances -the gain or transmission efciency of the channel |05 is decreased and the transmission efficiency of the channel |06 is increased in the manner previously explained. Accordingly, a substantial portion of the signal currents incoming to the substation circuit are transmitted through the channel |06 to the loud speaker |02 for audible reproduction. Following the initial break-in, the person using the substation circuit illustrated is expected to stop speaking. As a result, the control circuit |43 operates further to decrease the transmission eiciency of the channel and further to increase the transmission efciency of the channel |06 in a manner which will be clearly apparent from the foregoing explanation.`

Referring now more particularly to Fig. 2 of the drawings, the substation circuit there illustrated is, in general, quite similar to that illustrated in Fig. 1 and described above. Briefly described, this circuit comprises a transmitter or microphone 20|, receiving means in the form of a loud speaker 202, a hybrid system 203 including a line balancing network 204 and having line terminals 200, and two transmission channels 205 and 206 for coupling the microphone 20| and the loudspeaker 202 to the hybrid system 203,.

respectively. The circuit further comprises two control circuits. 242 and 243 which respectively correspond to the control circuits |42 and |43 illustrated in Fig. 1 and are provided for the purpose of controlling the transmission efficiencies of the two channels 205 and 206. The transmission channel 205 comprises a, coupling transformer 201, an adjustable signal level controlling voltage divider 2|2, an amplifier tube 2|4 and a voltage transforming network 2 l5 connected in tandem in the order named. The channel 206 similarly comprises a coupling transformer 228,

an adjustable voltage divider 23|, an amplifier tube 233, a voltage transforming circuit 235 and a coupling transformer 236 connected in tandem in the order named. More particularly, the input electrodes of the tube 2|4 are connected across the adjustable shunt leg 2|2b of the voltage divider 2| 2 in series with a C battery 2| 6' which is provided for biasing the control grid of the tube 2|4 to the proper negative potential with respect to the cathode of this tube. The output electrodes of the tube 2|4 are coupled to the outgoing signal current terminals of the hybrid system 203 through the voltage transformlng circuit 2|5, which circuit comprises a pair of resistors 2|1 and 2| B serially included in the channel and a third resistor'2l9 bridged across the channel. Screen and anode potentials are supplied to the tube 2|4 'from a source of direct current, not shown, but having its positive terminal connected to the terminal marked +B, `:his source being by-passed for signal frequency :urrents by a condenser 220. The amplier tube |33 included in the incoming signal current chaniel 206 comprises a pair of input electrodes which ire bridged across the adjustable shunt portion l3|b of the voltage divider 23|- in series with a- I battery 22| which functions to bias the conyrol grid of this tube to the proper negative po-4 ;ential with respect to the cathode thereof. The )utput electrodes of the tube 233 are coupled hrough the voltage transforming circuit 235 to .he primary winding of the transformer 236, a xy-pass condenser 222 being provided for byiassing .signal currents around the source of inode and screen potential which is connected to he terminal marked +B.

The two control circuits 2412 andl 243 are ofY dentical arrangement and for this reason only he details of the control circuit 243 have been ilustrated. In brief, this circuit comprises a pair if electron dscharge tubes 25| and 252 arranged I espectively to control the transmission eficiencies of the channels 206 and 205. More larticularly, the output electrodes of the tube 5| are bridged across the shunt leg 23|b of the oltage divider 23| through a low impedance in the rectifying circuit direct current blocking condenser 226 so that the transmission efficiency-of the channel 206 is variabledirectly in accordance with changes in the output electrode impedance of this tube. Similarly, the output electrodes of the tube 252 are bridged across the shunt leg 2|2b of the voltagedivider 2 I2 through a blocking condenser 221, whereby the signal transmission efficiency of the channel 205 is variable directly in accordance with the output electrode impedance of this tube. Anode potential is impressed upon the anode of the tube 252 over a path including the high im.. pedance iron core choke coil 228 and a resistor 229'. Similarly, anode potential is impressed upon the anode of the tube 25| overa path including ahigh impedance iron core choke coil 232' and a resistor 233'. As in the arrangement illustrated in Fig. 1, the output electrode impedances of the tubes 25| and 252 are controlled by varying the potentials impressed upon the control electrodes thereof. To this end, there is provided acircuit which includes a transformer 260 having its primary winding bridged across the channel 205 and its secondary windings in-v cluded in a rectifying circuit which also includes a pair of load resistors 26| and 262, the junction point of which is grounded, and a rectifier 263 which is preferably of the dry disk copper oxide type, g The two series connected resistors 26| and 262 are bridged by a smoothing condenser 264 so that pulsating direct current developed causes a resonably smooth direct voltage t be developed across the two load resistors. The voltages developed across the resistors 26| and 262 during signal current transmission through the channel 205 are utilized to control the potentials impressed upon the conthe other hand, is such that the control tube 25| is normally negatively biased to a point closely approaching anode current cut-olf, vwhereby the output electrode impedance of this tube is relatively high. With the rectifying circuit arranged in the manner illustrated, alternating signal currents traverse the rectifying circuit including the rectifier 263 and the resistors 262 and 26| in a. clockwise direction such that the right terminal of the resistor 262 is rendered more positive to decrease the bias impressed upon the control electrode' of the tube 25|, and the left terminal of the resistor 26| is rendered more negative such that the bias is increased upon the control electrode of the tube 252. Thus, when signal currents traverse the channel 205 the output electrode impedance of the tube 25| is decreased vided for varying the output electrode impedance f of the tube 252 inversely in accordance with the amplitude of signal currents impressed upon the input side of the channel 205. This automatic gain control circuit comprises a portion or all of the resistor 26|, an auxiliary rectifier 269,

preferably of the dry disk copper oxide type, and a coupling path including a direct current blocking condenser 210 and a resistor 21|.

As explained above, the 4control tubes 25| and 252 are normally biased to have high and low output electrode impedances respectively. Since the output electrode impedance of the tube 25| is bridged across the shunt leg 23| of the voltage divider 23| it will be apparent that the resistance of this shunt leg as combined with the output electrode impedance of the tube is relatively high corresponding to a relatively high transmission efficiency. On the other hand, the low output electrode impedance of the tube 252 which is connected to bridge the shunt leg 2|2b of the voltage divider 2|2 renders the transmission efficiency of the channel 205 relatively low for reasons which will be apparent from the foregoing explanation. By suitably adjusting the taps 2|3 and 232 any desired normal ratio between the transmission eiilciencies'of the two channels 205 and 206 may, within limits, be attained. With the apparatus at normal if a user of the substation circuit speaks into the microphone 20|, the generated voice currents are transmitted through the coupling transformer 201, the voltage divider 2|2, the amplifier tube 2|4, the volt- Vby means oi the condenser 210 and the resistor 21| upon the rectifying Icircuit which includes the rectifier 269 and the portion 26h. of the resistor 26|. The average amplitude of current traversing this rectifying circuit obviously varies directly in accordance with the average magnitude of the amplified signal currents.y The rectifier 269 is so Apoled that the current traversing the auxiliary rectifying circuit is in opposition to the current traversing the primary rectifying circuit which includes the rectifier 263. Thus, the

age transforming network 2|5 and the hybrid system 203 to the line extending to the'terminals 200.. If the hybrid system 203 is substantially unbalanced due to the conditions ofthe lines connected to the terminals 200, a substantial portion of the voice currents are transmitted" through this system to the input side of the channel 206. In the absence of the control cir.- cuit 243 such currents would be amplified in the channel 206 and delivered tov the'loud speaker 202 for reproduction. The indicated control cir- Y cuit, however, responds to the signalv'curr'e'nts traversing the channel 205 by substantially decreasing the transmission efiiciency of the channel 206 'and substantially increasing the trans"- mission efiiciency'of .the channel 205. More parnet voltage across the resistor 26| represents the difference between the voltage drops produced by the two currents. Since the current traversing the auxiliary rectifying circuit including the rectier 269 tends to vary with the input signal intensity, this net voltage varies with signal amplitude to change the bias and thus alter the output electrode impedance of the tube 252. More particularly, when the signal input tothe channel 205 is increased the currents traversing the auxiliary rectifyng circuit tend to increase to produce a corresponding decrease in the bias on the control electrode of the. tube 252 so that the output electrodeimpedance of this tube is decreased to lower the transmission efliciency of the channel 205 and. thus decrease the amplitude of the signal current delivered to the auxiliary rectifying circuit. Conversely, a decrease in the signal inputLto the microphone 20| is accompanied by a decrease in the magnitude of the signal currents impressed upon the auxiliary rectifying circuit so that an increase occurs in the biasing voltage of the tube 252, whereby the output electrode impedance of this tube is increased to produce an increase in the transmission eiiiciency of the 'channel 205. Preferably, the characteristics of the auxiliar-y rectifier 269 are so chosen that the automaticv gain control action just described is not started until the signal currents traversing 'the channel 205 reach an appreciable magnitude.

If a rectifier of the copper oxide or dry disk type is used, this may be accomplished by varying the ticularly, a portion-of the ampliiied signal curl rents'are delivered through the coupling transformer 260 to the rectifying circuit including the rectifier 263, whereby biasing potentials are developed across the load resistors 26| and 2 62. The condenser 26,4 is initially charged at an extremely high rate so that the biasing potentials are available immediately speech transmission through the channel 205 is started. As previously explained, the voltage developed across the resistor 262 is poled so that the normal bias impressed upon the control electrode of the tube 25| is lowered, thereby to decrease .the output impedance of this tube. Thus, the transmission efflciency of the channel 206 is decreased. Conversely, the voltage developed across the vresistor 26| functions to increase the negative bias upon the control electrode of the tube 252 so that the stantially nonlinear and no substantial current number of disks included therein, it being well understood in the art that the voltage-current characteristic `of a rectifier of this type is subfiow therethrough occurs until-a voltage of predetermined magnitude is impressed thereacross. but when this magnitude of voltage is exceeded the current rises steeply with increasing applied voltage. Hence, it will be understood that by varying the number of disks in the rectifier unit 269 no current flow through the circuit including output electrode impedance of this tube is inrectifier 269 functions to maintain the signal output level of the channel 205 substantially constant regardless of wide fluctuations in the intensity of the signal input to this channel. Thus, a portion of amplified signal voltage is impressed this rectifier will occur to start the automatic gain control action until the signal transmission level reaches a desired value.

The level of signal transmissionthrough the channel 205 is lowered somewhat by virtue of the inclusion of the voltagetransforming circuit 2|5 in this channel. Thus, the net signal voltage impressed across the outgoing signal current terminals of the hybrid system 203 is the difference between the signal voltage impressed across the primary winding of the coupling transformer 260 and thesignal voltage drops across the two series resistors 2|1 and 2|8 as determined by the value of the shunt resistor 2|9. The purpose of providing this voltage transformingcircuit is that of substantially reducing the magnitude of signal currents transmitted to the rectifying circuit including the rectifier 263 through the coupling afforded by unbalance in the hybrid system 203 when signal currents are transmitted to the substation circuit from a distant substation. The

phone 20|.

larger portion of signal currents incoming to the substation circuit over the associated line are transmitted through the hybrid system to the input side of the channel 206 and are delivered through the coupling transformer 228, the voltage divider 23|, the amplier tube 233, the voltage ltransforming circuit 235 and the coupling transever, these currents do not substantially affectl the vbiasing voltages of the control tubes 25| and 252 even under the most unfavorable conditions of unbalance. The control circuit 242 responds to the signal currents transmitted thereto over the channel 206 by operating to lower the transmission eciency of the channel 205 and to increase the transmission efficiency of the channel 206 in a manner identical with that just described with reference to the circuit 243. It will be noted that the input terminals of the control circuit 242 are coupled directly to the loud speaker 202, which speaker, being of the moving coil type, may function both as a transmitter and a receiver. Accordingly, sound waves impinging upon the diaphragm of this speaker cause signal voltages to be developed which are transmitted through the coupling transformer 236 and the voltage transforming circuit 235 to the input terminals of the control circuit 242. Due to the voltage reducing action of theA circuit 235, however, the magnitude of such currents is so low as to cause no substantial response on the part of the conitrol circuit 242.

The arrangement of Fig. 2, like that of Fig. 1, is

such that signal current transmission in either direction may be interrupted to permit conversation break-ins. Thus, if incoming signal currents are being transmitted through the hybrid system 203 and the channel 206 to the loud speaker 202 andthe user of the illustrated substation circuit desires to breakin upon the speech train, he may do so by speaking into the micro- 'I'he resulting signal currents are transmitted at low level through the voltage dlvider 2|2 to the input side of the amplifier 2M and following amplification through this tube are delivered through the coupling transformer 200 to the rectifying circuit including the rectifier 263. The resulting change in the output electrode impedance of the tubes 25| and 252 causes are transmitted through the hybrid system 203 to the channel 206 and the control circuit 242 responds thereto by operating to decrease the transmission efficiency of the channel 205 and to increase the.transmission efficiency of the channel 206.

AT he arrangement illustrated in Fig.. 3 of the drawings'is substantially similar' to that shown in Fig. 2 of the drawings and described above except that in the Fig. 3 arrangement the automatic gain control circuit has been omitted. Due to thes'imilarity of the two circuits corresponding elements thereof have been identified by reference characters having the same tens and units digits, but different hundreds digits, respectively corresponding to the figure numbers. The primary difference in the two ircuits resides in the arrangement of the rectifying circuit through operation of which the output electrode impedances of the two control tubes are controlled in magnitude. In the Fig. 3 arrangement the rectifying circuit includes a single resistor 36| which is shunted by a condenser 364 and connected in circuit with a rectifier 363 and the secondary winding of the coupling transformer 360, the junction point between the lower terminal of the secondary transformer winding and the upper pole of the rectifier 363 being grounded in the manner shown. With this arrangement, the voltage drop across the rectifier 363 functions as the voltage for controlling the bias potentials of the two control tubes 35| and 352, the rectier 363 beingso poled that an increased negative bias is impressed upon the.in' dicated control electrodeswhen signal currents traverse the channel 305. 'Ihis particular arrangement of the rectifying circuit is well adapted to the use of a vacuum tube diode as the rectifying element. Since; however, only a single control voltage of one polarity is available the circuit is arranged so that the output electrode impedances of the two control tubes 35| and 352 are changed in the same sense when a bias voltage is developed as a result of signal currents the transmission eiliciency of the channel 205 to be lowered and the transmission eciency of the channel 205 to be increased in the exact manner described above. The increase in the transmission eiciency of the channel 205 is such that the signal currents developed through operation of the microphone 20| are transmitted through this channel and the hybrid system 203 to the line connected to the terminals 200 with sulcient magnitude to cause the reproduction thereof by the receiving means provided at thedistant substation. Conversation break-ins may also occur during those intervals when signal currents are being transmitted from the microphone 20| through the channel 205 and the hybrid system 203 to the line connected to the terminals 200.

In the latter case the: incoming signal currents tions to control the transmission traversing the channel 305.

Accordingly, the output electrodes of the tube 35|, which funceflciency of the channel 306, are coupled to the series leg 33| of the voltage divider 33| so that when the output electrode impedance of the tube is increased the transmission efllciency of the channel 306 is decreased and vice versa. Since the two terminals of the'series leg 33|a are both above ground potential, a coupling transformer 375 is utilized to couple the output electrodes of the tube 35| across this leg of the voltage divider. 'I'he output electrodes of the tube 352,

Awhich is lprovided to control the transmission elciency of the channel 305, are bridged acrossl the' shunt leg 3|2b of the voltage divider 3|2 through the blocking condenser 321. ages of the two C batteries 365 and 366 are such that very low bias voltages are normally impressed upon the control electrodes of tubes 35| and 352. This, of course, means that the output electrode impedance of each of the two tubes is relatively low. With the output electrode impedance of the tube 35| at a'I low value, the transmission efficiency of the channel 306, as determinedby the control action of the tube 35|, is relatively high. Conversely, with the output electrode impedance of the tube 352 at a low value, the transmission eciency of the channel 305, as determined by the control action of this tubeis relatively low. By proper adjustthe twoA ment of the taps 3|3 and 332 the indicated transmission efciencies of the two channels may. be adjusted to any desired normal ratlo. With the apparatusat normal if-sound"waves impinge upon the diaphragm of the microphone 30|, the resulting signal currents are transmitted through the transformer 381, the voltage divider 3I2 and the tube 3I4 to the coupling transformer 360, thereby to cause rectified signal currents to traverse-the rectifying circuit including the rectifier 363. The voltage developed across the rectier 363 increases thenegative bias upon the control electrodes of each of the two tubes 35| and 352, thereby to Vcause the output electrode impedances of these two tubes to be increased.

As a result, the transmission emciency of the channel 305 is increased and the transmission eiiicienc'y oi' the channel 3061s decreased in a.

manner which will be clearly apparent from the foregoing explanation. Aside from the differences noted, the mode of operation of the circuit arrangement illustrated in Fig. 3 is exactly the same as that described above with reference to the arrangement illustrated in Fig. 2, it being noted, however, that no automatic gain control action is provided -in the Fig. 3 arrangecluding a series leg and a'shunt leg, a pair of electron discharge tubes each including an input electrode an `including output electrodes respectively bridged across the shunt legs of said voltage dividers, whereby the signal current transmission efficiencies of said channels`are respectively variable directly in accordance with the output electrode impedances of the respective associated tubes, means for impressing biasing potentials upon said control electrodes, a control circuit coupled to said one channel at one point and including signal voltage rectifying means and circuit elements for impressing the rectified pedances of said tubes being variable in accordance with changes in the potentials impressed upon the control electrodes thereof to vary the signal current transmission eiiiciencies of the respective associated channels, means responsive to signal currents traversing said one channel for varying the potentials upon said contol electrodes to increase the signal current transmission eiiciency of said one channel and to decrease the signal current transmission eiliciency of said other channel, and means including one of said tubes for varying the signal current transmission efficiency of said one channel inversely in accordance with the amplitude of the signal input thereto.

3. In a .transmission system, a pair of channels for transmitting signal currents in different directions, an electron discharge tube including a control electrode and a pair of output electrodes coupled to one of said channels, a second velectron discharge device including a control electrode and apair of output electrodes coupled to the other of said channels, the output 4electrode impedances of said tubes-being variable in accordance with changes in the potentials impressed upon the control electrodes thereof to vary the signal currenttransmission eliiciencies of the respective associated channels, means responsive to signal currents traversing said one channel for varying the potentials upon said control electrodes to increase the signal current transmission eihciency of said one channel and to decrease the signal current transmission efficiency of said other channel, and means controlled in accordance with the amplitude of signal currents traversing said one channel for further controlling the output electrode impedance of the tube associated with said one channel to vary the signal current transmission efficiency of said onechannel inversely in accordance with the amplitude of the signal input thereto.

4. In a transmission system, a signal current transmission channel, an electrondischarge tube including a pair of output electrodes, means for 'coupling said output electrodes to said lchannel so that the signal current transmission eiliciency of said channel is changed in one sense in response to an increase in the output electrode impedance of said tube and is changed in an opposite sense ,in response to a decrease in the output electrode impedance of said tube, means responsive to signal currents traversing said channel for changing the output electrode impedance of signal voltages upon said control electrodes to vary the potentials thereof in opposite senses such that the output electrode impedance of the tube associated with said one channel is increased and the output electrode impedance of the tube associated with said other channel is decreased when signal currents ltraverse said one channel, and means comprising a voltage transforming circuit included in said one channel at a point between said one point and said coupling means for preventing signal currents transmitted through said other channel and fed through said coupling means to said one channel from substantially changing said potentials.

2. In a transmission system, a pair of channels for transmitting signal currents in diierent directions, an electron discharge tube including a control electrode and a pair of output electrodes coupled to one of said channels, a second electron discharge device including a control electrode and a pair of output electrodes coupled to the other of said channels,- the output electrode imsaid tube substantially to increase the signal current transmission eiciency of said one channel, and means controlled in accordance with the amplitude of signal currents traversing said channel for further controlling the output electrode impedance of said tube to vary the signal current transmission eiiiciency of said channel inversely in accordance with the amplitude of the signal input thereto.

5. InV a transmission system, a signal current transmission channel, an electron discharge tube including a pair of output electrodes and a control electrode for changing the impedance across said output electrodes, means for coupling said output electrodes to said channel so that the signal current transmission eiciency of said channel is changed' in one sense in response to an increase in the output electrode impedance of said tube and is changed in an opposite sense in response to a decrease in the output electrode impedance of said tube, means for impressing a biasing potential upon said control electrode,

' transmission channel, an electron discharge tube including a pair oi' output electrodes and a control electrode for changing the impedance across said output electrodes, means for coupling said output electrodes to said channel so that the signal current transmission eiiciency. of said chan- Vnel is changed in one sense in response to an increase in the output electrode impedance of said tube and is changed in an opposite sense in response to a decrease in the output electrode impedance of said tube, means for impressing a biasing potential upon said control electrode, a control circuit coupled to said one channel and including signal voltage .rectifying means and a circuit element Afor impressing the rectied signal voltage upon said control electrode to vary the potential of said control electrode in a direction substantially to increase the signal current transmission eiliciency of said channel when signal currents traverse said channel, and a second control circuit including at least a portion of said circuit element and additional rectifying means for further controlling said potential to vary the Signal current transmission eiiiciency of said channel inversely in accordance withtheampli- `tude of the signal input thereto.

7. In a transmission system, a pair of channels for transmitting signal currents in different directions, a voltage divider included in'each of said channels, each of said-dividers including a series leg and a shunt leg, a pair of electron discharge `cies of said channels are respectively variable directly in accordance with the output electrode impedances of the respective associated tubes, means for impressing biasing potentials upon said control electrodes, means responsive to signal currents traversing one of said channels for varying said potentials in opposite senses such that the output electrode impedance of the tube associated with said one channel is increased and the output electrode impedance of the tube associated with said other channel is decreased, and means controlled in accordance with the amplitude of signal currents traversing said one channel for further controlling the output electrode impedance of the tube associated with said one of said tube and is changed in the opposite sense in response to a decrease in the output electrode limpedance of said tube, means responsive to signal currents traversing said channel for changing said impedance in a direction substantially to increase the signal current transmission eiliclency of said channel,` and means controlled in accordance with the amplitude of signal currents traversing said channel for further controlling said impedance to vary the signal current transmission emciencyof said channel inversely in accordance with the amplitude of the signal input thereto.

9. Telephone substation apparatus comprising transmitting means, receiving means and a hybrid system adapted to be connected to a line extending to said apparatus, a normally active channel for transmitting signal currents from said transmitting means to said hybrid system, a second normally active channel for transmitting signal 'currents from said hybrid system to said receiving means, a voltage divider included in each v of said channels, each of said dividers including a leg serially included in the associated channel and a leg shunting a portion of the associated channel, an electron discharge tube including output electrodes bridged across one leg of the divider included in one of said channels so that the signal current transmission efficiency of said one channel is determined by the impedance across said electrodes, a second electron discharge tube including output electrodes bridged across one leg of the voltage divider included in the V .mission eliiciency of said one channel and substantially to decrease the signal current transmission e'iciency of said other channel, and means controlled in accordance with the amph- Vtude of signal currents traversing said one channel for changing the output electrode impedance of said first-named tube to vary the signal current transmission elciency of said one channel inversely in accordance with the amplitude of the signallinput thereto.

l0. In a transmission system, a, signal current transmission channel, an electron discharge tube including a pair of output electrodes and a control electrode for changing the impedance across said output electrodes, means coupling said output electrodes to said channel so that the transchannel to vary the signal current transmission efficiency of said one channel inversely "in'accordance with the amplitude of the signal input' 8. In a. transmission system, a signal current transmission channel, a voltage divider comprising a leg serially included in said channel and a. leg shunting a portion of said channel, an electron discharge tube including a pair of output electrodes bridged across one of said legs, whereby the signal current transmission efciencv of said channel is changed in one sense in response to an increase in the output electrode impedance rectly in accordance with the amplitude of the signal input to said channel, whereby the transmission eiciency of said channel is varied inversely in accordance with the amplitude of the signal input thereto. Y

- ROSWELL H. HERRICK. 

